What are the main electromagnetic interference sources of switching power supply module

1. Harmonic distortion of input rectifier circuit

The input end of switching power supply generally adopts bridge rectifier and capacitor filter circuit. The rectifier bridge can only be turned on when the pulsating voltage exceeds the voltage on the input filter capacitor, and the current is input from the voltage power supply and charges the filter capacitor battery. If the voltage on the filter capacitor is higher than the instantaneous speed voltage of the overvoltage power supply, the rectifier will stop. Therefore, the current of the input circuit is single pulse, and has a variety of high-efficiency harmonic currents. Due to the discrete system characteristics of the rectifier circuit, the current on the AC side of the rectifier bridge is seriously out of frame.

The harmonic frequency on the DC side is n times. Therefore, the high-frequency harmonic current on the DC side of the rectifier circuit will not only cause the output power of the circuit and increase the reactive power of the circuit, but also cause transmission distortion and radiation source distortion along the transmission route.

2. Dry deflection caused by switch circuit

Switching circuit plays an important role in switching power supply, and it is also one of the main interference sources. The switch load is the primary coil of high-frequency transformer, which is an inductive load. At the moment of conduction, the primary coil causes a large inrush current, and a high surge peak voltage appears on both sides of the primary coil; At the moment of disconnection, due to the leakage flux of the primary coil, part of the kinetic energy is not transmitted from the multiple electromagnetic coils to the secondary electromagnetic coils. A part of the kinetic energy stored in the inductor will produce attenuation fluctuations with spikes with the capacitors and resistors in the collector circuit, which will be accumulated on the turn-off voltage to produce turn-off voltage spikes. If the spike has any high force, it is very likely to thermally puncture the switch tube